1. Field of the Invention
The present invention relates to a rolling bearing, and more particularly to a rolling bearing which is suited to supporting a photoconductor drum incorporated in information equipment for electrophotographic processing, such as copiers (including digital PPCs and color PPCs) and printers (including color LBPs and color LED printers).
2. Description of the Related Art
FIG. 11 shows the general configuration of an electrostatic transfer copier. In this copier, an electrostatic latent image is formed on a photoconductor drum 21 with electric charges according to the intensity variations of light 22. A developing drum 23 applies toner having a charge of reversed polarity onto the latent image to form a visible image. The visible image is transferred to a sheet of paper fed from a paper feeding unit 24, and the sheet is then forwarded to a fixing unit 25 in which the toner is fixed to the sheet under heat and pressure. The fixing unit 25 is chiefly composed of a fixing roller 26 having a heater thereinside, and a pressure roller 27 arranged in tight contact with the fixing roller 26 in parallel.
In general, the photoconductor drum 21 is rotatably supported at both ends by rolling bearings such as ball bearings and rolling bearings. These bearings tend to be charged by the electrification of the photoconductor drum 21 during the toner application. The bearing charges, if left standing, can affect the charge distributions of the drum and toner in reverse, possibly causing image disorders and the like. Therefore, some type of grounding mechanism is required. As a conventional countermeasure thereto, an independent grounding mechanism has been provided on an end face of the photoconductor drum.
Nevertheless, the provision of an independent grounding mechanism as described above increases the number of parts in the periphery of the photoconductor drum and adds constraints to the design flexibility. On this account, a structure has been proposed in which a current carrying function is added to a bearing itself so as to release the charges to exterior (outer ring side). This structure, however, requires electric connection between the inner and outer rings which are insulated from each other, and therefore necessitates the use of a so-called current-carrying bearing incorporating current-carrying grease, current-currying seals, or other current-carrying parts. The current-carrying bearings have been proposed, for example, in Japanese Utility Model Laid-Open Publication No. 1992-8820, Japanese Patent Laid-Open Publication No. 1999-82492, and Japanese Patent Laid-open Publication No. 1995-71451. All these bearings, however, use special parts or special materials not used in conventional bearings, and therefore cost more.
In view of the foregoing, an object of the present invention is to provide a rolling bearing having excellent current-carrying capability at low cost.
To achieve the foregoing object, the present invention provides a rolling bearing comprising an inner ring, an outer ring, and a plurality of rolling elements arranged between the inner and outer rings, oil films being interposed between the rolling elements and raceway surfaces of the rings, wherein the inner raceway surface or the outer raceway surface comes into metal contact with the rolling elements for electric connection. This electric connection can be realized, for example, by making either of the raceway surfaces rougher than in ordinary products (surface roughening).
The insulation between the inner and outer rings in an ordinary rolling bearing is ascribable to the interposition of a lubrication oil film between the inner ring and the rolling elements or between the rolling elements and the outer ring. However, a lubrication oil film is indispensable in maintaining favorable bearing functions. Then, either of the raceway surfaces is formed with a surface roughness higher than conventional as mentioned above, so that the inner ring and the rolling elements, or the rolling elements and the outer ring make frequent metal contact with each other. This establishes almost continuous electric connection therebetween, greatly reducing the average resistance of the bearing to realize a current-carrying. state. Here, the metal contact occurs only in limited small areas within the entire contact region (favorable lubrication oil films are formed in the remaining majority of areas), and the contacting time is extremely short. Therefore, poor lubrication and other adverse effects resulting from the metal contact can also be avoided. The surface roughening to the rolling contact surface can be achieved in such a way that the grinding step after cutting is omitted or simplified. In this case, the reduction of steps cuts down costs. The grinding step after heat treatment can be omitted to allow color-based discrimination from ordinary articles (unground articles have black surfaces).
Specifically, the present invention provides) a rolling bearing comprising an inner ring, an outer ring, and a plurality of rolling elements arranged between the inner and outer,rings, oil films being interposed between the rolling elements and raceway surfaces of the rings, wherein the inner raceway surface or the outer raceway surface is equal to or higher than 0.1 xcexcm in surface roughness Ra. This is considerably higher than a conventional bearing""s surface roughness (Ra=0.02 xcexcm or so).
The surface roughness Ra employed herein refers to center line average height (JIS B0601). This expresses in micrometer the value given by the equation of FIG. 10, where a portion having a measurement length of L is extracted from a roughness curve along the direction of its center line, and the roughness curve is represented by y=f(x) with the center line of the extracted portion and the direction of longitudinal magnification on the x-axis and the y-axis, respectively. A roughness curve is a curve obtained from a profile curve by a measuring method having the characteristic of removing low frequency components. A center line is such a straight or curved line parallel to an average line that the areas enclosed by this line and the roughness curve become the same at both sides of this line. An average line is a straight or curved line which has the nominal shape of the surface to be measured and is drawn so that the sum of squares of deviations between this line and the profile curve is minimized.
The rolling bearing according to the present invention may be applied to a ball bearing and a roller bearing. In ball-bearing applications, the above-mentioned surface roughness Ra of the inner or outer raceway surface is set so that the surface roughness Ra (xcexcm) and an oil film thickness (xcexcm) calculated by Archard-Cowking equation have a ratio [surface roughness/oil film thickness] of 0.3 or higher. Meanwhile, in roller-bearing applications, the surface roughness Ra of the inner or outer raceway surface is set so that the surface roughness Ra (xcexcm) and an oil film thickness (xcexcm) calculated by Grubin equation have a ratio [surface roughness/oil film thickness] of 0.3 or higher.
The rolling bearings described above are suited to supporting a photoconductor drum in information equipment for electrophotographic processing. Since this type of applications involve mild load conditions and relatively lower revolutions, adverse effects resulting from the metal contact can be avoided.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.